Since the advent of xerography, the trend in the copier arts has been to construct copiers with ever increasing speed and accuracy, while simultaneously providing a greater level of functionality to the end user. For the early part of copier's history, that trend was marked by incremental performance improvements through better engineering of materials and design.
The first copiers were slow and cumbersome for the user--document pages were manually fed in one sheet at a time, thus demanding the undivided attention of the human user. This situation improved for the user with the introduction of the automatic document feeder. The automatic feeder, coupled to, and synchronized with a "light/lens" scanner, allowed for faster copying with correspondingly less attention required of the user.
The input scanner of the typical copier, for many years, was the traditional "light/lens" scanning bar. The scanning bar worked by illuminating a portion of a document page and projecting that image, through a series of optics, to a photoreceptor which then captured the image. The scanning bar would proceed down the page, line-by-line fashion, until the entire image was captured.
Although light/lens scanning continues to provide reliable image capture, a major limitation is speed. Scanning a document page, line-by-line, by mechanically moving the scan bar down the length of the document page has become the bottleneck in document processing. To alleviate this bottleneck, some have attempted to flash expose a document page in order to capture the entire page image in one short instance. One such imaging system is described in commonly assigned U.S. Pat. No. 4,466,734 to Rees, entitled "Compact Full Frame Illumination and Imaging System for a Photocopier", issued Aug. 21, 1984. Rees describes a compact, full-frame illumination system employing a movable lens system that allows for image enlargement and reduction. Rees' system does not, however, capture the document image in a digital format.
With the increasing demand for image manipulation and processing, this lack of digital image capture represents another limitation of light/lens scanning. Light/lens scanners may perform some limited image processing quite naturally--namely, image reduction and enlargement. These processes are easily accomplished by selecting the correct optical magnification.
However, with users becoming more sophisticated in terms of the range of applications desired, a digital format is more amenable to these sophisticated demands. For example, image rotation, identification and selection, optical character recognition, and the like are more easily performed using a digital image format.
To meet the demand for digital image processing, a number of linear digital image scanners were developed: charge coupled devices (CCD), butted silicon full width arrays, amorphous silicon full width arrays, and raster input scanners. Although all of the above mentioned scanners capture a document's image digitally, they all suffer the same defect of capturing an image line-by-line scanning. As with the early light/lens systems, the limitation of capturing an image by line scanning is speed. Capturing a portion of the document at a time is not as fast as capturing the entire, full-page image simultaneously.
Another limitation arises due to the relative motion that occurs between the document and the linear scanning bar. This relative motion happens if the document is passed over the linear scanning bar or if the bar is moved over the length of the document. In either case, exacting mechanical precision is required to accurately capture the image of the entire document. Even the slightest deviation in motion control may cause visual effects in the final hard copy printout that is noticeable to the human eye. Moreover, relative motion problems may be exacerbated at very high throughput rates.
In a copier, the problems of speed and relative motion arise not only at the input image capture (where the image of the document page is captured), but also where an image derived from digital data exposes a photoreceptor belt for ultimate transfer to a hard copy medium such as paper. For example, one method of producing a latent image from digital data is raster output scanning. A raster output scanner sweeps a laser beam across the surface of a moving photoreceptor and writes the image in a line-by-line fashion. This linear writing method exhibits both drawbacks of speed and relative motion control.
Another method of digital printing that eliminates relative motion control problems has been described in: "Hybrid ER Copier", "Printer Modification for Flash Based Copier", "Color Printer Modification for Flash Based Copier", and "Fax Modification for Flash Based Copier", all by Thomas Hammond, published in Xerox Disclosure Journal (XDJ) Vol. 16, No. 5, September/October 1991, at pages 309, 311, 305 and 307 respectively.
Hammond's basic digital printer, as described in "Printer Modification for Flash Based Copier", uses a reflective (i.e. no backlighting), full frame, liquid crystal display (LCD) as a front end image projector for a flash based copier. Hammond proposes positioning the LCD over the platen of the copier in such a manner as to allow the LCD to provide the input image into the copier.
To print digital data, Hammond first loads digital data into the LCD. Once the full LCD page is displayed, the copier would flash expose the LCD, thus exposing the copier's photoreceptor with the image data displayed on the LCD. In effect, the photoreceptor would be exposed with the same image data as if the original document were on the platen.
Hammond's basic digital copier, as described in "Hybrid ER Copier", again uses a reflective LCD display sitting atop a platen of a flash based copier. Additionally, Hammond adds a raster input scanner (RIS) and an appropriate electronic subsystem (ESS) to his digital printer configuration. In copy mode, a document is scanned by the RIS and the information is processed by the ESS and loaded into the LCD array a page at a time. After the LCD is set, operation continues in the same fashion as Hammond's digital printer described above.
Hammond's method of digital printing/copying allows for the decoupling of the functions of digital printing/copying and ordinary light/lens copying. Hammond describes that, by removing the LCD from atop the platen, the flash-expose copier may function in an independent, stand-alone mode. However, merely combining an LCD and a separate copier together to produce a printer is not effective from both a design and user point of view for several reasons.
First, to achieve this effective decoupling, Hammond uses a reflective LCD (as opposed to a transmissive LCD) to provide image capture. The copier then performs image exposure of its photoreceptor by flash exposing the LCD whenever the user commands. To copy another full frame image, the user would wait for the next image to be set in the LCD before the copier is allowed to flash expose. Thus, without a tighter coupling of the LCD to the copier, printer/copier throughput suffers.
Second, a separate, detachable LCD would need to be properly registered every time it was reattached to the copier. This reattachment and registration requires that the user invest additional time in the proper usage of Hammond's digital printer and copier.
Third, in Hammond's digital copier, the image input is still produced by a line-by-line, raster scanning technique. It is known that raster scanning, however, is limited by the top speed that a laser beam can be scanned across a page line. Additionally, raster scanning must be exactingly precise to eliminate relative motion errors.
Thus, there is a need for a digital imager for a copier that tightly couples the functions of image capture and image exposure. Additionally, there is a need for a digital imager that can synchronize these functions at electronic speeds to increase printer/copier throughput without the attendant problems of relative motion.
Therefore, it is an object of the present invention to create an imager that captures a full-page document image digitally while simultaneously eliminating the motion control problems associated with image capture and exposure.
It is another object of the present invention to create an imager that electronically couples the functions of image capture and image exposure on to a photoreceptor surface.